The present invention relates to a fluorescent substance for use in Stellus type bar code. More particularly, the invention relates to modification of a parent material structure and formulation of infrared-emitting fluorescent substance having an emission spectrum in the infrared wavelength region, and the formation of fine particles of the fluorescent substance.
Currently, article control by the use of bar codes, is actively adopted in industry, and particularly the circulation industry. Further, bar codes are printed on prepaid cards, passing cards, etc., and are read out by means of an optical reading device such as scanner or the like. Further, a variety of forgery-preventing means are applied to credit cards and the like, and a variety of means for detecting forgery on these cards have been proposed. As one of such means, there has been proposed an optical reading device in which a semiconductor laser is irradiated to a latent image mark or the like printed with a fluorescent substance-containing ink to excite the fluorescent substance and the light emitted from the fluorescent substance is detected as bar code information.
According to this method, a forged or altered card can be discovered with certainty because a fluorescent signal is emitted only when a recorded mark is present. Further, forgery or alteration of the card is quite difficult to practice because the contents of the latent image are unknown to persons other than the true card maker.
Fluorescent materials for Stellus type bar code such as LiNdYbP.sub.4 O.sub.12, NdYbP.sub.5 O.sub.14, LiNdP.sub.4 O.sub.12, KNd(PO.sub.4).sub.2, NaNd(WO.sub.4).sub.2 and the like have been used (Japanese Patent Publication No. 40594/1978).
These fluorescent substances are called stoichio metric fluorescent substances, and the cations in these substances are exclusively composed of rare earth elements such as Nd, Yb, etc. Accordingly, this type of fluorescent substance have a problem that it has a deep blue color due to the presence of Nd.
Further, this type of fluorescent substance has the problem of large particle size which can be 7 .mu.m or more, so that it has to be pulverized when applied to offset the printing process or ink ribbon, and the pulverization deteriorates crystallinity and formulation of fluorescent substance and much decreases the fluorescence intensity thereof.
Further, since rare earth elements are generally expensive, the use of a large quantity of such rare earth element is one of the causes of the high price of fluorescent substances.
Further, at the present time, the self flux process is adopted for the manufacture of these fluorescent substances, and the adoption of such a process also increases the price of the fluorescent substances. Further, the self flux process is low in product yield, and therefore enhances the price of fluorescent substances. The self flux process is a process of adding the starting phosphate material in a largely excessive amount to rare earth elements and separating the excessive phosphate from fluorescent substance by a treatment using nitric acid after firing.